Magnetic recording media typically comprise a thin magnetic layer carried on a non-magnetic support. The magnetic layer typically comprises a magnetic pigment dispersed in a resinous binder or a magnetic coating deposited directly onto the non-magnetic support. Types of magnetic media include video tape, audio tape, diskette, computer tape, stretched surface media and many other configurations of a magnetic layer bound to a non-magnetic support. The present invention is particularly concerned with magnetic tapes, and most particularly with video tapes.
The magnetic recording process involves converting an electric signal current into an equivalent magnetization on a magnetic tape. This is done with a transducer, known in the art as a head, that transforms the electric signal into a magnetic field, through which a tape passes. The magnetic particles of the tape are then left with a remanence representative of the field which can be read by a head to reproduce the original signal. A typical video recorder uses a rotary head/drum assembly configuration known as a helical scanner.
The characteristics which relate to the ability of a tape to reproduce a signal precisely are referred to as its electromagnetic properties. The art continually strives for improving the electromagnetic properties of a tape. Previously, the art has concentrated on improving electromagnetics through improvements in the magnetic layer.
An additional desired property of a tape is its ability to keep deposits from building up on the head. In an effort to combat the problem of head build-up, the art has typically added head cleaning agents to the magnetic coating layer of the tape. Head cleaning agents are typically non-magnetic particles which have a higher degree of abrasivity than the magnetic coating layer.
It is desirable to provide a tape having substantially improved electromagnetic properties which utilizes presently known magnetic layers. It is further desired to provide a tape which exhibits improved head cleaning characteristics without additional head cleaning agents being added to the magnetic layer.
The present invention provides a tape which achieves greatly improved electromagnetics and head cleaning properties in a magnetic recording tape by utilizing a non-magnetic support film which has a very high Young's modulus in the transverse direction relative to the Young's modulus in the longitudinal direction.
The Young's modulus, or tensile modulus, of a film is a measure of the film's stiffness and is determined according to the standard method defined in ASTM D-882-11.7, extrapolated method. The tensile modulus is a ratio of the stress over strain in the initial linear portion of the stress-strain curve and is reported in Kg/mm.sup.2. The tensile modulus is calculated by multiplying the force, extrapolated from the curve to 1% elongation, by 100 and dividing by the original (nominal) cross-sectional area of the sample.
For very narrow films, such as an audio or video tape, it is very difficult and inaccurate to measure tensile modulus in the above manner. Thus, the flexural modulus is measured and converted into tensile modulus. Tensile modulus and flexural modulus are related by a constant in the range of small deformations. The flexural modulus of a tape is determined according to the standard method defined in ASTM D79O.7l and is a measure of the deflection of the center of the tape as a force is applied.
It has now been discovered that in a series of tapes in which the Young's modulus in the transverse direction is increased relative to the Young's modulus in the longitudinal direction a point is reached where it is believed a configurational change occurs in the tape as it moves over the head. This configurational change allows the tape to more closely approach or contact the center gap region of the head resulting in greatly improved electromagnetics and head cleaning action.
In U.S. Pat. No. 4,316,927, filed Feb. 2, 1979 ('927 patent), it is taught that a tape having a ratio of Young's modulus in the longitudinal direction to that in the transverse direction of between 0.5 and 0.9 provides a tape which exhibits improved head contact. The '927 patent further teaches that when this ratio is decreased below 0.5 to 0.4 the head contact is decreased, and thus, ratios of below 0.5 should be avoided (col. 6, lines 30-45). In contrast, the present inventors have discovered that as the ratio decreases to approximately 0.5 a configurational transformation occurs as the tape passes over the head and the electromagnetics of the tape improve substantially from a ratio of about 0.5 and continuing to a ratio of about 0.3. The '927 patent did not recognize this but instead teaches that a ratio of 0.5 is the minimum useful value.
Further, the present invention provides improved head cleaning properties. It is believed that prior art tapes have edge contact with the head leaving spacing between the center of the tape and the head gap. This allows deposits to collect and remain on the crucial gap portion of the head adversely affecting performance. The tape of the present invention provides contact between the tape and the center of the head, thereby providing cleaning action on the gap portion of the head.
In U.S. Pat. No. 4,318,957 ('957 patent), filed Dec. 26, 1978, carrier foils (non-magnetic support films) for magnetic tapes having Young's moduli higher in the transverse direction than in the longitudinal direction are described. The '957 patent teaches that these tapes are more resistant to buckling, folding, wrinkling, cupping and the like. These configurational phenomena are manifested as a tape passes over a pin or guide. The '957 patent teaches that the ratio of Young's modulus in the longitudinal direction to Young's modulus in the transverse direction should be less than 0.77 (inverse of the 1.3 actually recited in '957) and preferably less than 0.61 (inverse of 1.65 recited in '957). The '957 patent clearly does not teach or appreciate the surprising results obtained by the practice of the present invention which occur below a ratio of about 0.5 and continuing to a ratio of about 0.3.